Radeon Pro 580 and Blender 2.8
Its not the GPU's Fault. More like Apple's support for OpenGL/CL.

the RX 580s run very well under Windows and Linux. You can either use dualboot or a Virtualmachine using HW Passtthru to use blender thru for example Linux with direct Hardware access to the gpu.

Regarding the second Question: You should mostly never render to Video directly. You may try a newer 2.8 Build or a different codec.